BELGRADE, Yugoslavia Portions of southern Yugoslavia have reached such a state of ethnic friction that Yugoslavs have begun to talk of the horrifying possibility of ''civil war'' in a land that lost one-tenth of its population, or 1.7 million people, in World War II. The current hostilities pit separatist-minded ethnic Albanians against the various Slavic populations of Yugoslavia and occur at all levels of society, from the highest officials to the humblest peasants. A young Army conscript of ethnic Albanian origin shot up his barracks, killing four sleeping Slavic bunkmates and wounding six others. The army says it has uncovered...